1. Field of Invention
This invention relates in general to cardiac monitoring and in particular to dynamic estimation of fluid responsiveness.
2. Related Art
Indicators and methods for noninvasive determination of fluid status of patients are important for real-time monitoring of the condition of critical care patients.
Numerous studies have demonstrated that pulse pressure variation (PPV) is one of the most sensitive and specific predictors of fluid responsiveness. Specifically, PPV has been shown to be useful as a dynamic indicator to guide fluid therapy in different patient populations receiving mechanical ventilation. For instance, PPV was found to exhibit better performance as a predictor of fluid responsiveness in patients before off-pump coronary artery bypass grafting than standard static preload indexes. PPV has also been shown to be useful for predicting and assessing the hemo-dynamic effects of volume expansion and a reliable predictor of fluid responsiveness in mechanically ventilated patients with acute circulatory failure related to sepsis. Another study concluded that PPV can be used to predict whether or not volume expansion will increase cardiac output in postoperative patients who have undergone coronary artery bypass grafting. A critical review of studies investigating predictive factors of fluid responsiveness in intensive care unit patients concluded that PPV and other dynamic parameters should be used preferentially to static parameters to predict fluid responsiveness.
The standard method for calculating PPV often requires simultaneous recording of arterial and airway pressure. Pulse pressure (PP) is calculated on a beat to beat basis as the difference between systolic and diastolic arterial pressure. Maximal PP (PPmax) and minimal PP (PPmin) are calculated over a single respiratory cycle, which is determined from the airway pressure signal. Pulse pressure variations ΔPP are calculated in terms of PPmax and PPmin and expressed as a percentage,
                              P          ⁢                                          ⁢          P          ⁢                                          ⁢                      V            ⁡                          (              %              )                                      =                  100          ×                                                    PP                max                            -                              PP                min                                                                    (                                                      PP                    max                                    +                                      PP                    min                                                  )                            /              2                                                          (        1        )            
Despite the usefulness of PPV, automatic determination of PPV is a difficult problem and it is often very difficult to accurately determine this parameter automatically in regions of abrupt hemodynamic changes such as operating room conditions or regions with artifact. Respiratory variations in arterial pulse pressure calculated manually (PPVman) are accurate predictors of fluid responsiveness in mechanically ventilated patients. However, they cannot be continuously monitored. Thus, it is not possible o to conveniently monitor this manual index in the operating room or in the intensive care unit. Commercially available systems capable of monitoring PPV do not work well in regions of abrupt hemodynamic changes.
As a consequence, other simpler methods for prediction of fluid responsiveness have been proposed. These methods have the advantage that can be obtained by direct application of a simple formula to the arterial blood pressure signal. For example, U.S. Pat. No. 7,422,562 discloses a method for determining a cardiac parameter equal to or derivable from cardiac stroke volume variation (SVV) comprising: inputting a waveform data set corresponding to arterial blood pressure over a computation interval that covers at least two cardiac cycles; calculating a standard deviation value for the waveform data set over each cardiac cycle; and calculating an estimate of the SVV as a function of the standard deviation values.
The ideal method should be a predictive of fluid responsiveness as PPV but significantly easier to calculate and provide accurate information about fluid status in operating room conditions.